The present invention relates to monolithic ceramic capacitors mountable on electronic circuit boards. More specifically, the invention concerns ceramic chip capacitors of a variable, free form shape composed of multi layer polycrystalline ceramic and metal composites and the method of making these capacitors.
The basic model of a capacitor, as shown in FIG. 1, is a single plate device 10 consisting of two electrodes 12, 14 insulated from each other by dielectric material 16. FIG. 2 illustrates a basic monolithic ceramic chip capacitor 20. According to known technique, a monolithic ceramic capacitor 20 is manufactured by interleaving electrode layers 22, 24 and dielectric layers 26, compressing all layers, and sintering the layers to form a solid monolithic block.
The principal characteristic of a capacitor is that it is capable of storing electric charge, and this feature is useful in a variety of applications, including, discharge of stored energy, blockage of direct current, coupling of circuit components, by-passing of an AC signal, frequency discrimination and transient voltage and arc suppression. A capacitor's ability to store electric charge is directly proportional to the capacitance value of the capacitor and the voltage applied to it. Accordingly, to optimize the performance of a capacitor, it is desirable to maximize the capacitor's capacitance value.
For any given voltage, the capacitance value of a device is directly proportional to the area and inversely proportional to the thickness, this relationship is expressed as EQU C=K(A/d)
where:
C is capacitance PA1 K is Constant dependent on the medium between the two conductors PA1 A is area PA1 d is thickness.
As is evident by the relationship stated above, greater capacitance value can be achieved either by increasing the electrode area or choosing a dielectric material with a high relative permittivity.
In general, capacitors can utilize a variety of dielectrics such as gas (or vacuum), naturally occurring elements (e.g., mica) or prepared materials (e.g., ceramics). Chip capacitors usually utilize ceramic dielectric materials based on the titanates or niobates. The most common dielectric is barium titanate. Numerous attempts have been made to optimize the electrical properties of these dielectric materials by developing new processes and materials. These efforts, however, are costly and time consuming.
An alternative method of optimizing a capacitor's value is to increase the area of the capacitor. Traditionally, chip capacitors used for surface mounting on electronic circuits have been rectangular in shape. Some attempts have been made to create disk-shaped, crown-shaped or honeycomb-shaped capacitors. These capacitors, however, are costly and difficult to manufacture. Consequently, due to the ease of manufacturability, most capacitors are limited to a rectangular, square or circular shape.